Logic circuit with overdriven off-state switching

ABSTRACT

A logic circuit includes: a main switching means for changing conduction state between at least two terminals in accordance with a voltage supplied to a control terminal; and a voltage converting means for converting a voltage at an input terminal and outputting the converted voltage to the control terminal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates, as is indicated, to a logiccircuit. More particularly, the present invention relates to a logiccircuit for high speed operation at low power supply voltage.

2. Description of the Related Art

As described in, for example, Japanese Laid-open Patent Publication No.5-14166, increasing speed of operation has been tried in recent years bysupplying a voltage higher than the power supply voltage between a gateand a source of a transistor using a capacitor. FIG. 31 is a diagram ofa logic circuit of the prior art. Reference labels 3101, 3120 and 3110designate a logic input terminal, an output terminal and a power supplyfor supplying a voltage of Vdd, respectively. Reference labels 3108 and3109 designate a P-channel FET (field effect transistor) and anN-channel FET, respectively. Reference labels 3106 and 3107 designatebias power supplies for supplying a voltage of Vs. Reference labels 3102and 3103 designate capacitors. Reference labels 3104 and 3105 designatean N-channel FET and a P-channel FET, respectively.

However, the logic circuit shown in FIG. 31 suffers from the followinginherent limitations. That is to say, a leak current flows when the FETs3108 and 3109 are in an OFF state. Furthermore, it is not possible inthe prior art to sufficiently drive (i.e., overdrive) the FETs 3108 and3109, or to sufficiently out off the FETs 3108 and 3109 by biasing itsgate with deep backward bias voltage. Therefore, the logic circuitaccording to the prior art results in a high dissipation power andcannot perform a high-speed operation at a low voltage.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a logic circuitincludes: a main switching means for changing the conduction statebetween at least two terminals in accordance with a voltage supplied toa control terminal; and a voltage converting means for is converting avoltage at an input terminal and outputting the converted voltage to thecontrol terminal.

In one embodiment of the invention, the voltage converting meansincludes a voltage generating means and a sub switching means.

In another embodiment of the invention, the sub switching means includesan SOI (semiconductor-on-insulator) structure.

In still another embodiment of the invention, the voltage generatingmeans includes at least one of a capacitor; a battery and a highdielectric material.

In still another embodiment of the invention, the voltage convertingmeans supplies a higher voltage than that of the input terminal to thecontrol terminal in a first state, the first state being a state of theinput terminal where the main switching means is conductive.

In still another embodiment of the invention, the voltage convertingmeans supplies a lower voltage than that of the input terminal to thecontrol terminal in a second state, the second state being a state ofthe input terminal where the main switching means is non-conductive.

In still another embodiment of the invention, the voltage convertingmeans supplies a higher voltage than that of the input terminal to thecontrol terminal in a first state; and the voltage converting meanssupplies a lower voltage than that of the input terminal to the controlterminal in a second state; the first state being a state of the inputterminal where the main switching means is conductive; and the secondstate being a state of the input terminal where the main switching meansis non-conductive.

In still another embodiment of the invention, the voltage convertingmeans further includes a capacitor of which a first terminal isconnected to the input terminal; and wherein the sub switching meansconnects a second terminal of the capacitor to the control terminal ofthe main switching means in the first state; and connects the secondterminal of the capacitor to the voltage generating means, and connectsthe control terminal of the main switching means to the ground in astate other than the first state.

In still another embodiment of the invention, the voltage convertingmeans further includes a capacitor of which a first terminal isconnected to the input terminal; and wherein the sub switching meansconnects a second terminal of the capacitor to the control terminal ofthe main switching means in the second state; and connects the firstterminal of the capacitor to the control terminal of the main switchingmeans, and connects the second terminal of the capacitor to the groundin a state other than the second state.

In still another embodiment of the invention, the voltage convertingmeans further includes a first capacitor of which a first terminal isconnected to the input terminal and a second capacitor of which a firstterminal is connected to the input terminal; and wherein the subswitching means connects a second terminal of the first capacitor to theground and connects a second terminal of the second capacitor to thecontrol terminal of the main switching means in the first state; andconnects the second terminal of the first capacitor to the controlterminal of the main switching means and connects the second terminal ofthe second capacitor to the voltage generating means in the secondstate.

In still another embodiment of the invention, the main switching meansincludes an N-channel MOS FET.

In still another embodiment of the invention, the main switching meansincludes a P-channel MOS FET.

In still another embodiment of the invention, the main switching meansincludes a complementary MOS FETs.

According to another aspect of the invention, a logic circuit includes:a first and a second main switching means for changing conduction statebetween at least two terminals in accordance with a voltage supplied toa control terminal; a first voltage converting means for converting avoltage at an input terminal and outputting the converted voltage to thecontrol terminal of the first main switching means; and a second voltageconverting means for converting a voltage at an input terminal andoutputting the converted voltage to the control terminal of the secondmain switching means, wherein the first voltage converting meansincludes a first capacitor having a first terminal and a secondterminal, a second capacitor having a first terminal and a secondterminal, a first sub switching means, and a first voltage generatingmeans; the first terminal of the first capacitor and the first terminalof the second capacitor being connected to the input terminal; thesecond voltage converting means includes a third capacitor having afirst terminal and a second terminal, a fourth capacitor having a firstterminal and a second terminal, a second sub switching means, and asecond voltage generating means; the first terminal of the thirdcapacitor and the first terminal of the fourth capacitor being connectedto the input terminal through an inverter; the first sub switching meansconnects a second terminal of the first capacitor to the ground andconnects a second terminal of the second capacitor to the controlterminal of the first main switching means in the first state; andconnects the second terminal of the first capacitor to the controlterminal of the first main switching means and connects the secondterminal of the second capacitor to the first voltage generating meansin the second state; and the second sub switching means connects thesecond terminal of the first capacitor to the ground and connects thesecond terminal of the second capacitor to the control terminal of thefirst main switching means in the second state; and connects the secondterminal of the first capacitor to the control terminal of the secondmain switching means and connects the second terminal of the secondcapacitor to the second voltage generating means in the first state.

Thus, the invention described herein makes possible the advantage ofproviding a logic circuit which enables a high-speed operation at a lowpower supply voltage, and in which a leak current (i.e., a dissipationpower) is small. For at least these advantages, the logic circuitaccording to the present invention contributes to increasing operationspeed and reducing dissipation power, especially for a battery-poweredportable equipment.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a logic circuit 1 according to thepresent invention.

FIGS. 2A-2C are diagrams of a first example of the logic circuitaccording to the present invention.

FIG. 3 is a diagram of the first example of the logic circuit accordingto the present invention using an N-channel FET and P-channel FETs.

FIG. 4 is a diagram illustrating voltages of nodes 301-304 in FIG. 3 instates I and II.

FIG. 5A is a diagram of a logic circuit for substituting a P-channel FETin place of the N-channel FET used as the main switching device 220 inFIG. 2A.

FIG. 5B is a diagram of a logic circuit according to the presentinvention using N-channel FETs and a P-channel FET as the two switches.

FIGS. 6A-6C are diagrams of a second example of the logic circuitaccording to the present invention.

FIG. 7 is a diagram of the second example of the logic circuit accordingto the present invention using an N-channel FET and P-channel FETs.

FIG. 8 is a diagram illustrating voltages of nodes 701-704 in FIG. 7 instates I and II.

FIGS. 9A-9C are diagrams of a third example of the logic circuitaccording to the present invention.

FIG. 10 is a diagram of the third example of the logic circuit accordingto the present invention using an N-channel FET and P-channel FETs.

FIG. 11 is a diagram illustrating voltages of nodes 1001-1005 in FIG. 10in states I and II.

FIG. 12 is a diagram of a circuit configuration for substituting inplace of the main switching device 1020 shown in FIG. 10.

FIG. 13 is a diagram of a fourth example of the logic circuit accordingto the present invention.

FIG. 14 is a diagram of a logic circuit according to the presentinvention using FETs as the four switches in FIG. 13.

FIG. 15 is a diagram of the fifth example of the logic circuit accordingto the present invention

FIG. 16 is a diagram of a logic circuit according to the presentinvention using FETs as the four switches in FIG. 15.

FIG. 17 is a diagram of a sixth example of the logic circuit accordingto the present invention.

FIG. 18 is a diagram of a seventh example of the logic circuit accordingto the present invention.

FIG. 19 is a diagram of a eighth example of the logic circuit accordingto the present invention.

FIG. 20 is a diagram illustrating a structure of an N-channel MOS FETand its equivalent circuit.

FIG. 21 is a diagram illustrating a structure of a P-channel MOS FET andits equivalent circuit.

FIG. 22A is a diagram illustrating a part of the first example of thelogic circuit.

FIG. 22B is a diagram illustrating a equivalent circuit shown in FIG.22A.

FIG. 23 is a diagram illustrating a structure of an N-channel transistorand a P-channel transistor both having an SOI structure.

FIG. 24A is a diagram illustrating a transfer gate type switch.

FIG. 24B Is a graph showing a conductance Gon between a drain and asource of the transfer gate as a function of a source voltage Vs.

FIG. 25 is a graph showing a conductance Gon between a drain and asource of the transfer gate as a function of a source voltage Vs whenthe source voltage is low.

FIG. 26 is a graph showing a polarization of the capacitor of whichinsulating material is high dielectric constant material as a functionof a supplied voltage.

FIG. 27 is a circuit diagram of a logic circuit of the prior art to becompared.

FIGS. 28A and 2B are graphs showing voltages Va and Vc of the logiccircuit of the prior art, and voltages Va and Vc of the logic circuit ofthe present invention, respectively at the rising edge of the outputsignal.

FIGS. 28C and 28D are graphs showing voltages Vb and Vc of the logiccircuit of the prior art, and voltages Vb and Vc of the logic circuit ofthe present invention, respectively at the falling edge of the outputsignal.

FIG. 29 is a circuit diagram of the logic circuit of the presentinvention.

FIG. 30A is a graph showing voltages Va and Vb of the logic circuit ofthe present invention at the rising edge of the output signal.

FIG. 30B is a graph showing a gate voltage Vg of the main switchingdevice and an output voltage Vout of the logic circuit of the prior artat the rising edge of the output signal.

FIG. 30C is a graph showing voltages Va and Vb of the logic circuit ofthe present invention at the falling edge of the output signal.

FIG. 30D is a graph showing a gate voltage Vg of the main switchingdevice and an output voltage Vout of the logic circuit of the prior artat the falling edge of the output signal.

FIG. 31 is a diagram of a logic circuit of the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail with reference tothe drawings, wherein like elements are referred to with like referencelabels throughout. In this specification, "V" followed by a referencelabel representing a node designates a voltage potential of the nodewith respect to the ground. For example, a "voltage V1" represents avoltage potential of the "node 1" from the ground level. For the sake ofsimplicity, a voltage potential of a node with respect to the groundlevel is referred to as a "voltage of the node".

In this specification, in an ideal state, a logic low level(hereinafter, referred to as an "L (low) level") is equal to 0 V (i.e.,the ground potential), a logic high level (hereinafter, referred to asan "H (high) level") is equal to a voltage potential higher than that ofthe ground by a power supply voltage Vdd. Ideally, a node of a logiccircuit (especially for an output node) should be either of an H leveland an L level, should not be at an intermediate potential level betweenthe H level and the L level. In an actual logic circuit, however, avoltage of a node reads an intermediate voltage level in an transientstate.

FIG. 1 is a schematic diagram of a logic circuit 1 according to thepresent invention. The logic circuit 1 includes a voltage converter 10and a main switching device 20. The voltage converter 10 drives the mainswitching device 20 in accordance with a voltage of node 11.

The main switching device 20 is an active device having nodes 21-23, anda conduction state between nodes 22 and 23 changes in accordance with avoltage of the nodes 21. In this specification, a state wherein aswitching device conducts a current is referred to as an "ON" state, anda state wherein a switching device does not conduct a current isreferred to as an "OFF" state. As the main switching device 20, forexample, an FET (field effect transistor) can be used. When an FET isused as the main switching device 20, nodes 21, 22 and 23 correspond toa gate, a drain and a source, respectively.

Node 14 is supplied with a power supply voltage Vdd. A resistor 13 pullsup node 12, which functions as an output terminal of the logic circuit1, to the power supply voltage Vdd. When the main switching device 20 isON state, node 12 is at the L level. When the main switching device 20is OFF state, node 12 is at the H level. In FIG. 1, although nods 22 ispulled up, and node 23 is connected to the ground, the configuration isnot limited to this. For example, as the main switching device 20, aplurality of switching device which are connected so as to constitute atransfer gate can be utilized.

The voltage converter 10 converts a voltage of node 11 and then outputsthe converted voltage to node 21, thereby improving characteristics ofthe main switching device 20. These improvements include severalaspects: increasing a switching speed (i.e., an improvement in transientcharacteristics), and reducing a ON-state resistance of a switchingdevice (i.e., an improvement in static characteristics). The voltageconverter 10 used in the logic circuit 1 according to the presentinvention has three types: Types 1-3.

In Type 1, when node 11 is at the L level (i.e., the ground level), node21 is at the ground level, and when node 11 is at the H level (i.e., theVdd level), node 21 is at a voltage level of (Vdd+Vov1). Here, Vdd andVov1 designate positive voltage values. Therefore, in the logic circuit1 of Type 1, when node 21 is at the H level, i.e., the main switchingdevice 20 is ON state, the main switching device 20 is overdriven. Inother words, when the main switching device 20 is ON state, node 21which in a control terminal of the main switching device 20 1s suppliedwith a higher voltage than Vdd. This enables reduction in a turn-on timeof the main switching device 20, and reduction in a turn-on resistance(i.e., a resistance corresponding to a drain-source voltage VDS(sat) inON state) of the main switching device 20. As the main switching device20 used in the logic circuit 1 of Type 1, an FET of which thresholdvoltage Vt is equal to 0.35 V or more is preferably used. The thresholdvoltage Vt is a gate-source voltage Vgs when a drain current Id is zero,which is also referred to as a pinch-off voltage.

In Type 2, when node 11 is at the L level (i.e., the ground level), node21 is at a level of -Vov2, and when node 11 is at the H level (i.e., theVdd level), node 21 is at a voltage level of Vdd. Here, Vov2 designatespositive voltage values. Therefore, in the logic circuit 1 of Type 2,when node 21 is at the L level, i.e., the main switching device 20 isOFF state, the main switching device 20 is cut off sufficiently. Inother words, when the main switching device 20 is OFF state, node 21which is a control terminal of the main switching device 20 is suppliedwith a lower voltage than the ground level. This enables reduction in aturn-off time of the main switching device 20, and increasing a turn-offoff resistance of the main switching device 20. As the main switchingdevice 20 used in the logic circuit 1 of Type 2, an FET of whichthreshold voltage Vt is equal to 0.35 V or less is preferably used.

In Type 3, when node 11 is at the L level (i.e., the ground level), node21 is at a level of -Vov2, and when node 11 is at the H level (i.e., theVdd level), node 21 is at a voltage level of (Vdd+Vov1). Therefore, inthe logic circuit 1 of Type 3, when node 21 is at the H level, i.e., themain switching device 20 is ON state, the main switching device 20 isoverdriven. In other words, when the main switching device 20 is ONstate, node 21 which is a control terminal of the main switching device20 is supplied with a higher voltage than Vdd. This enables reduction ina turn-on time of the main switching device 20, and reduction in aturn-on resistance (i.e., a resistance corresponding to a drain-sourcevoltage VDS(sat) in ON state) of the main switching device 20.

In the logic circuit 1 of Type 3, when node 21 is at the L level, i.e.,the main switching device 20 is OFF state, the main switching device 20is cut off sufficiently. In other words, when the main switching device20 is OFF state, node 21 which is a control terminal of the mainswitching device 20 is supplied with a lower voltage than the groundlevel. This enables reduction in a turn-off time of the main switchingdevice 20, and increasing a turn-off resistance (i.e., a resistancecorresponding to a drain-source voltage VDS(sat) In ON state) of themain switching device 20. As the main switching device 20 used in thelogic circuit 1 of Type 3, an FET of which threshold voltage Vt is equalto 0.35 V or less is preferably used.

EXAMPLE 1

FIGS. 2A-2C are diagrams of a first example of the logic circuitaccording to the present invention. The logic circuit of the firstexample is classified into the above Type 1.

The operation of the logic circuit shown in FIG. 2A will be describedbelow. The logic circuit shown In FIG. 2A includes a voltage converter210 and a main switching device 220. Node 211 receives an input signalfrom an external device, thereby being set to either of the H level andthe L level. In this specification, a state wherein a main switchingdevice (designated by the reference label 220 in the first example) isOFF is referred to as "state I", and a state wherein a main switchingdevice is ON is referred to as "state II". In state II, the voltageconverter 210 outputs to node 221 a voltage of (Vdd+Vov1) higher thanthe voltage Vdd which is supplied to node 211. As a result,characteristics of the main switching device 220 is improved. As themain switching device 220, a MOS (metal-oxide-semiconductor) FET isused, and node 221, 222 and 223 are a gate, a drain and a source,respectively. In all the examples including the first example, as themain switching device, an SOI (semiconductor-on-insulator) transistor,an MES (metal-semiconductor) FET, a bipolar transistor, a TFT (thin filmtransistor) and the like can be used.

The voltage converter 210 includes switches S20 and S21, a voltagegenerator 214, and a capacitor 212. Switches S20 and S21 can beconstituted using a plurality of FETs. Although a power supply voltageVdd is used as the voltage generator 214 in the first example, a voltagevalue supplied by the voltage generator 214 is not limited to Vdd.Although an external power supply is used of the voltage generator 214in the first example, the type of the voltage generator 214 is notlimited to this. As the voltage generator in all examples in thespecification, a capacitor, a battery and the like can be used. As acapacitor, such as the capacitor 212 and a capacitor as the voltagegenerator 214, a capacitor using material of a high dielectric constantis preferably used for the reason described in detail later.

In state I, the switch S20 is placed in a position such that contact C1is connected to contact C2, while in state II, the switch S20 is placedin a position such that contact C1 is connected to contact C3. In stateI, the switch S21 is placed in a position such that contact C1 isconnected to contact C2, while in state II, the switch S21 is placed ina position such that contact C1 is not connected to contact C2. FIG. 2Acorresponds to state I. FIG. 23 is a diagram representing an equivalentcircuit of the logic circuit shown in FIG. 2A in state I. FIG. 2C is adiagram representing an equivalent circuit of the logic circuit shown inFIG. 2A in state II.

As shown in FIG. 2B, in state I, node 211 is at the L level. In state I,node 211, to which one terminal of the capacitor 212 is connected, isconnected to the ground, while the other terminal of the capacitor 212is connected to the voltage generator 214. As a result, the capacitor212 is charged by the voltage Vdd supplied by the voltage generator 214.In state I, node 221 which is a gate of the main switching device 220 isconnected to the ground, whereby electric charge in the gate of the mainswitching device 220 in discharged. An a result, the turn-off time ofthe main switching device 220 is shortened.

As shown in FIG. 2C, In state II, node 211 is at the H level. In stateII, node 211, to which one terminal of the capacitor 212 is connected,is pulled up to the power supply voltage Vdd, while the other terminalof the capacitor 212 is connected to node 221. As a result, node 221which is a gate of the main switching device 220 is supplied with avoltage of (Vdd+Vov1). Here, Vov1 is a positive voltage and is governedby the voltage supplied by the voltage generator 214 and a gate-sourcecapacitance of the main switching device 220. A relationship C212>>Cgsis preferably satisfied, where C212 and Cs are a capacitance of thecapacitor 212 and a capacitance of the gate-source capacitance of themain switching device 220, respectively.

FIG. 3 is a diagram of the first example of the logic circuit accordingto the present invention using an N-channel FET and P-channel FETs. Avoltage converter 310 and a main switching device 320 correspond to thevoltage converter 210 and the main switching device 220, respectively.An inverter 330 inverts a voltage supplied to node 301 and then outputsto node 302. In other words, node 302 is at the L level when node 301 isat the H level, while nods 302 is at the H level when node 301 is at theL level. A capacitor 312 and a voltage generator 314 correspond to thecapacitor 212 and the voltage generator 214, respectively. FETs 332 and334 function as the switch S20, and an FET 336 functions as the switchS21. The main switching device 320 corresponds to the main switchingdevice 220, and nodes 304, 322 and 323 correspond to nodes 221, 222 and223, respectively. The logic circuit shown in FIG. 3 operates in thesame manner as discussed above with respect to FIGS. 2A-2C with theexception that the main switching device 320 is OFF state when node 301is at the H level, while the main switching device 320 is ON state whennods 301 is at the L level (i.e., a logic of node 301 is inverted). Thusnode 302 corresponds to node 211. The same is also true with respect toother examples in this specification. Therefore, using of an iverter perse is not an essential part of the present invention, rather theinverter is necessary to switch switching devices in the voltageconverter at the predetermined phase. Therefore, further details areomitted.

FIG. 4 is a diagram illustrating voltages of nodes 301-304 in FIG. 3 instates I and II. A voltage V304 of node 304, which is a gate of the mainswitching device 320, is (Vdd+Vov1) in state II. Due to the voltageV304, the main switching device 320 is sufficiently driven. As a result,this circuit topology enables reduction in the turn-on time and theturn-on resistance.

Here, a drain current Id is given by the following equation as afunction of a gate-source voltage Vgs of an N-channel MOS translatorused as the main switching device 320:

    Id=K(Vgs-Vt).sup.2,

where K is a positive constant, Vt is a threshold voltage of theN-channel FET. In the near future, an operation voltage of an LSI (largescale integrated circuit) will become lower and lower for highreliability and low power dissipation. It is assumed that a power supplyvoltage Vdd is 1.0 V, the threshold voltage Vt is 0.35 V, and thevoltage generator 314 supplies the power supply voltage Vdd. A ratio ofthe drain current of the main switching device 320 in the logic circuitaccording to the present invention to a drain current in a logic circuitaccording to the prior art is as follows:

    (1.0+1.0-0.35).sup.2 /(1.0-0.35).sup.2 =6.44.

As will be appreciated from the above expression, the present inventionallows the drain current of the main switching device 320 to increase,thereby enabling a higher operation.

FIG. 5A is a diagram of a logic circuit for substituting a P-channel FETin place of the N-channel FET used as the main switching device 220 inFIG. 2A. FIG. 5B is a diagram of a logic circuit according to thepresent invention using N-channel FETs and a P-channel FET as the twoswitches. A voltage converter 510 and a main switching device 520correspond to the voltage converter 210 and the main switching device220, respectively. Node 511, 521-523 correspond to node 211, 221-223,respectively. Switches S50 and S51, a capacitor 512, and a voltagegenerator 514 correspond to the switches S20 and S21, the capacitor 212,and the voltage generator 214, respectively. A power supply 516 suppliesan output voltage to node 523 through the main switching device 520.Although the voltage generator 514 supplies the same voltage value as avoltage of Vdd supplied by the power supply 516, however, the voltagelevel is not limited to Vdd.

The logic circuit shown in FIG. 5A is described below. In state I, theswitch S50 is placed in a position such that contact C1 is connected tocontact C2, while in state II, the switch S50 is placed in a positionsuch that contact C1 is connected to contact C3. In state I, the switchS51 is placed in a position such that contact C1 is connected to contactC2, while in state II, the switch S51 is placed in a position such thatcontact C1 is not connected to contact C2. In state I, one terminal ofthe capacitor 512, which is connected to node 511, is at the H level,while the other terminal of the capacitor 512, which is connected tocontact C1 of the switch S50, is at the ground level. Therefore, instate I, the capacitor 512 is charged. On the other hand, the gate ofthe main switching device 520 (i.e., node 521) is set to be at the Hlevel.

In state II, node 511 will go down to the L level, and the otherterminal of the capacitor 512 will be connected to node 521. As aresult, node 521 is supplied with a voltage of -Vov1 (Vov1: a positivevoltage).

In FIG. 5B, FET 550 corresponds to the switch S51, and FETs 551 and 552correspond to the switch S50. A signal "D bar" supplied to the gate ofthe FET 550 is a signal obtained by inverting the signal D. The logiccircuit shown in FIG. 5B is a complementary circuit to the logic circuitshown in FIG. 3, and its operation in similar to that of the logiccircuit of FIG. 3.

EXAMPLE 2

FIGS. 6A-6C are diagrams of a second example of the logic circuitaccording to the present invention. The logic circuit of the secondexample is classified into the above Type 2.

The operation of the logic circuit shown in FIG. 6A will be describedbelow. The logic circuit shown In FIG. 6A includes a voltage converter610 and a main switching device 620. Node 611 receives an input signalfrom an external device, thereby being set to either of the H level andthe L level. In state I, the voltage converter 610 outputs to node 621 avoltage of -Vov2 lower than the ground level (i.e., 0 V) which issupplied to node 611. As a result, characteristics of the main switchingdevice 620 is improved. As the main switching device 620, a MOS(metal-oxide-semiconductor) FET is used, and node 621, 622 and 623 are agate, a drain and a source, respectively.

The voltage converter 610 includes switches S60 and S61, and a capacitor612. Switches S60 and S61 can be constituted using a plurality of FETs.Although contact C3 of the switch S60 is connected to the ground in thesecond example, a voltage value supplied to contact C3 of the switch S60is not limited to this ground potential. As will be appreciated from theoperation described below, however, contact C3 of the switch S60 ispreferably connected to the ground.

In state I, the switch S60 is placed in a position such that contact C1is connected to contact C2, while in state II, the switch S60 is placedin a position such that contact C1 is connected to contact C3. In stateI, the switch S61 is placed in a position such that contact C1 is notconnected to contact C2, while in state II, the switch S61 is placed ina position such that contact C1 is connected to contact C2. FIG. 6Acorresponds to state I. FIG. 6B is a diagram representing an equivalentcircuit of the logic circuit shown in FIG. 6A in state I. FIG. 6C is adiagram representing an equivalent circuit of the logic circuit shown inFIG. 6A in state II.

As shown in FIG. 6B, in state I, node 611 is at the L level. In state I,node 611, to which one terminal of the capacitor 612 is connected, isconnected to the ground, while the other terminal of the capacitor 612is connected to node 621. As a result, node 621 which is the gate of themain switching device 620 is supplied with a voltage -Vov2. Here, Vov2is a positive voltage and is governed by the voltage supplied to node611 and a gate-source capacitance of the main switching device 620. Arelationship C612>>Cgs is preferably satisfied, where C612 and Cgs are acapacitance of the capacitor 612 and a capacitance of the gate-sourcecapacitance of the main switching device 620, respectively.

As shown in FIG. 6C, in state II, node 611, to which one terminal of thecapacitor 612 is connected, is at the H level, while the other terminalof the capacitor 612 is connected to the ground. As a result, thecapacitor 612 is charged by the voltage Vdd supplied by an externaldevice connected to node 611.

FIG. 7 is a diagram of the second example of the logic circuit accordingto the present invention using an N-cannel FET and P-channel FETs. Avoltage converter 710 and a main switching device 720 correspond to thevoltage converter 610 and the main switching device 620, respectively.An inverter 730 inverts a voltage supplied to node 701 and then outputsto node 702. In other words, node 702 is at the L level when node 701 isat the H level, while node 702 is at the H level when node 701 ia at theL level. A capacitor 712 corresponds to the capacitor 612. FETs 732 and734 function as the switch S60, and an FET 736 functions as the switchS61. The main switching device 720 corresponds to the main switchingdevice 620, and nodes 704, 722 and 723 correspond to nodes 621, 622 and623, respectively. The logic circuit shown in FIG. 7 operates in thesame manner as discussed above with respect to FIGS. 6A-6C. Therefore,further details are omitted.

FIG. 8 is a diagram illustrating voltages of nodes 701-704 in FIG. 7 instates I and II. A voltage V704 of node 704, which is a gate of the mainswitching device 720, is -Vov2 in state I. Due to the voltage V704, themain switching device 720 is sufficiently cut off. As a result, thiscircuit topology enables reduction in the turn-off time and increase inturn-off resistance.

EXAMPLE 3

FIGS. 9A-9C are diagrams of a third example of the logic circuitaccording to the present invention. The logic circuit of the thirdexample is classified into the above Type 3.

The operation of the logic circuit shown in FIG. 9A will be describedbelow. The logic circuit shown in FIG. 9A includes a voltage converter910 and a main switching device 920. Node 911 receives an input signalfrom an external device, thereby being set to either of the H level andthe L level. In state I, the voltage converter 910 outputs to node 921 avoltage of -Vov2 lower than the ground level (i.e., 0 V) which issupplied to node 911. In state II, the voltage converter 910 outputs tonode 921 a voltage of (Vdd+Vov1) higher then a voltage of Vdd which issupplied to node 911. As a result, characteristics of the main switchingdevice 920 is improved. As the main switching device 920, a MOS(metal-oxide-semiconductor) FET is used, nd nods 921, 922 and 923 are agate, a drain and a source, respectively.

The voltage converter 910 includes switches S90 and S91, a voltagegenerator 914, and capacitors 912 and 913. Switches S90 and S91 can beconstituted using a plurality of FETs. Although a power supply voltageVdd is used as the voltage generator 914 in the third example, a voltagevalue supplied by the voltage generator 914 is not limited to Vdd.Although an external power supply is used as th voltage generator 914 inthe third example, the type of the voltage generator 914 is not limitedto this. As the voltage generator 914, for example, a capacitor usingmaterial of a high dielectric constant can be used.

In state I, the switch S90 is placed in a position such that contact C1is connected to contact C2, while in state II, the switch S90 is placedin a position such that contact C1 is connected to contact C3. In stateI, the switch S91 is placed in a position such that contact C1 isconnected to contact C2, while in state II, the switch S91 is placed ina position such that contact C1 is connected to contact C3. FIG. 9Acorresponds to state I. FIG. 9B is a diagram representing an equivalentcircuit of the logic circuit shown in FIG. 9A in state I. FIG. 9C is adiagram representing an equivalent circuit of the logic circuit shown inFIG. 9A in state II.

As shown in FIG. 9B, in state I, node 911 is at the L level. In state I,node 911, to which one terminal of the capacitor 912 is connected, isconnected to the ground, while the other terminal of the capacitor 912is connected to the voltage generator 914. As a result, the capacitor912 is charged by the voltage Vdd supplied by the voltage generator 914.In state I, node 921 which is a gate of the main switching device 920 isconnected to the ground through the capacitor 913. The capacitor 913, aswill be described later, is charged in state II, thereby supplying avoltage of Vov2 between nodes 911 and 921. Therefore, a voltage of node921 will be Vov2. Here, Vov2 is a positive voltage and is governed bythe voltage supplied to node 911 (i.e., Vdd) and a gate-sourcecapacitance of the main switching device 920. A relationship C913>>Cgsis preferably satisfied, where C913 and Cgs are a capacitance of thecapacitor 913 and a capacitance of the gate-source capacitance of themain switching device 920, respectively.

As shown in FIG. 9C, in state XI, node 911 is at the H level. In stateII, node 911, to which one terminal of the capacitor 913 is connected,is pulled up to the power supply voltage Vdd, while the other terminalof the capacitor 913 is connected to the ground. As a result, thecapacitor 913 is charged by the power supply voltage Vdd. In state II,node 921 which is the gate of the main switching device 920 is connectedto node 911 through the capacitor 912. The capacitor 912, as discussedabove, is charged in state I, thereby supplying a voltage of Vov1between node 911 and 921. Therefore, a voltage of node 921 is(Vdd+Vov1). Here, Vov1 is a positive voltage and is governed by thevoltage supplied by the voltage generator 914 (i.e., Vdd) and agate-source capacitance of the main switching device 920. A relationshipC912>>Cgs is preferably satisfied, where C912 and Cgs are a capacitanceof the capacitor 912 and a capacitance of the gate-source capacitance ofthe main switching device 920, respectively.

FIG. 10 is a diagram of the third example of the logic circuit accordingto the present invention using an N-channel FET and P-channel FETs. Avoltage converter 1010 and a main switching device 1020 correspond tothe voltage converter 910 and the main switching device 920,resistively. An inverter 1030 inverts a voltage supplied to node 1001and then outputs to node 1002. In other words, node 1002 is at the Llevel when node 1001 is at the H level, while node 1002 is at the Hlevel when node 1001 is at the L level. Capacitors 1012 and 1013, and avoltage generator 1014 correspond to the capacitors 912 and 913, and thevoltage generator 914, respectively. FETs 1032 and 1034 function as theswitch S90, and an FET 1036 functions as the switch S91. The mainswitching device 1020 corresponds to the main switching device 920, andnodes 1005, 1022 and 1023 correspond to nodes 921, 922 and 923,respectively. The logic circuit shown in FIG. 10 operates in the samemanner as discussed above with respect to FIGS. 9A-9C. Therefore,further details are omitted.

FIG. 11 is a diagram illustrating voltages of nodes 1001-1005 in FIG. 10in states I and II. A voltage V1005 of node 1005, which is a gate of themain switching device 1020, is -Vov2 in state I and (Vdd+Vov1) in stateII. Due to the voltage V1005, the main switching device 1020 issufficiently driven and sufficiently cut off. As a result, this circuittopology enables reduction in the turn-on time, the turn-off time andthe turn-on resistance, and increase in the turn-off resistance.

FIG. 12 is a diagram of a circuit configuration for substituting inplace of the main switching device 1020 shown in FIG. 10. Node 1205corresponds to node 1005 and receives the output from the voltageconverter 1010. Node 1225 is pulled up to the power supply voltage Vdd.FETs 1220 and 1221 turn on alternately, thereby changing the voltage ofnode 1222. In other words, the FET 1220 is OFF and the FET 1221 is ONwhen node 1205 is at the H level, whereby node 1222 will become the Llevel. The FET 1220 is ON and the FET 1221 is OFF when node 1205 is atthe L level, whereby node 1222 will become the H level. The voltageconverter in the logic circuit of the third example outputs the voltageof -Vov2 (<0) in state I, and outputs the voltage of (Vdd+Vov1) (>Vdd),thereby efficiently driving the main switching devices having theconfiguration of the series connection of the complementary FETs shownin FIG. 12.

Referring again to FIG. 9A, it is assumed that the voltage generator 914generates the power supply voltage Vdd, and that node 911 at the H levelis equal to the power supply voltage Vdd. Accordingly, the capacitors912 and 913 are charged by the voltage Vdd. However, the chargingvoltage is not limited to Vdd. In order to supply a higher voltage tothe control terminal (i.e., the gate terminal), thereby overdriving themain switching device, a voltage which the voltage generator 914generates is preferably high. Although contact C3 of the switch S91 isconnected to the ground, contact C3 can be set to a different voltagelevel. In order to deeply cut off the main switching device, contact C3is preferably supplied with a lower voltage potential than the groundlevel.

EXAMPLE 4

FIG. 13 is a diagram of a fourth example of the logic circuit accordingto the present invention. The fourth example includes the circuittopology as discussed above with respect to the first example and thesecond example. Initially, an operation of a voltage converter 1310 andan FET 1331 will be described below. A state wherein the FET 1331 is OFFstate is referred to as state I, and a state wherein the FET 1331 is ONstate is referred to as state II. In state I, a switch 1312 is placed ina position such that a capacitor 1311 is connected to a voltagegenerator 1313, and a switch 1315 is placed in a position such that agate of the FET 1331 is connected to a power supply 1380. In state II, aswitch 1312 is placed in a position such that a capacitor 1311 isconnected to the FET 1331, and a switch 1315 is placed in a positionsuch that the gate of the FET 1331 is not connected to a power supply1380. As a result, in state I, the gate of the FET 1331 is supplied witha voltage potential lower than the ground level (i.e., 0V). This voltagepotential corresponds to the voltage of the -Vov2 as discussed in thesecond example.

Next, an operation of a voltage converter 1320 and an YET 1332 will bedescribed below. A state wherein the FET 1332 is OFF state is referredto as state I, and a state wherein the FET 1332 is ON state is referredto as state II. In state I, a switch 1322 is placed in a position suchthat a capacitor 1321 is connected to a voltage generator 1323, and aswitch 1325 is placed in a position such that a gate of the FET 1332 isconnected to the ground. In state II, a switch 1322 is placed in aposition such that a capacitor 1321 is connected to the FET 1332, and aswitch 1325 is placed in a position such that the gate of the FET 1332is not connected to the ground. As a result, in state II, the gate ofthe FET 1332 is supplied with a voltage potential higher than the powersupply voltage Vdd. This voltage potential corresponds to the voltage of(Vdd+Vov1) as discussed In the first example.

In the fourth example, the power supply 1380 supplying the power supplyvoltage Vdd, and the power generators 1313 and 1323 are used. The powergenerators 1313 and 1323 generate a voltage of, for example, Vdd. Inplace of the power generators 1313 and 1323, the power supply 1380 canbe used. In such case, a contact which is connected to a negativeelectrode of the voltage generator 1313, is connected to the ground; anda contact which is connected to a positive electrode of the voltagegenerator 1323, is connected to a positive electrode of the power supply1380. Such circuit topology requires only one kind of the power supply(i.e., power supply 1380). The some is true with respect to the otherlogic circuits according to the present invention as discussed in theother examples.

Node 1390 is at the L level when node 1301 is at the H level, while Node1390 is at the H level when node 1301 is at the L level. In other words,the logic circuit shown in FIG. 13 functions as an inverter.

FIG. 14 is a diagram of a logic circuit accords ing to the presentinvention using FETs as the four switches in FIG. 13. FETs 1412 and 1414correspond to the switch 1312, and FETs 1422 and 1424 correspond to theswitch 1322. FETs 1415 and 1425 correspond to the switches 1315 and1325, respectively. The logic circuit shown in FIG. 14 operates in thesame manner as discussed referring to FIG. 13. Therefore, furtherdetails are omitted.

EXAMPLE 5

FIG. 15 is a diagram of the fifth example of the logic circuit accordingto the present invention. The logic circuit shown in FIG. 15 has similarconfigurations to the logic circuit of FIG. 13 with the exception thatthe FETs 1331 and 1332 are replaced with FETs 1531 and 1532, and thatnode 1301 is replaced with nodes 1501 and 1502. The switches 1312, 1315,1322 and 1325 operate in the same manner as discussed above with respectto FIG. 13.

Assuming that node 1502 receives an input signal D, node 1501 receives Dbar as an input signal. FETs 1531 and 1532 turn on and turn offsimultaneously, thereby changing conduction state between nodes 1591 and1592. For example, when nodes 1501 and 1502 are at the L level and the Hlevel, respectively, the FETs 1531 and 1532 are ON state. On the otherhand, when nodes 1501 and 1502 are at the H level and the L level,respectively, the FETs 1531 and 1532 are OFF state. In other words, inthe fifth example, the logic circuit according to the present inventionis applied to a transfer gate.

FIG. 16 is a diagram of a logic circuit according to the presentinvention using FETs as the four switches in FIG. 15. The logic circuitshown in FIG. 16 operates in the same manner as discussed referring toFIG. 15. Therefore, further details are omitted.

EXAMPLE 6

FIG. 17 is a diagram of a sixth example of the logic circuit accordingto the present invention. In the logic circuit shown in FIG. 17, thevoltage converter 1010 in the third example described with respect toFIG. 10 driven FETs 1731 and 1732. When node 1701 is at the L level,voltages of nodes 1705 and 1706 are (Vdd+Vov1) and -Vov2, respectively,whereby nodes 1791 and 1792 are conduction state. When node 1701 is atthe H level, voltages of nodes 1705 and 1706 are -Vov2 and (Vdd+Vov1),respectively, whereby nodes 1791 and 1792 are in a non-conduction state.

According to the sixth example of the present invention, when FETs 1731and 1732 are OFF state, nodes 1705 and 1706 are respectively suppliedwith -Vov2 and (Vdd+Vov1). This results in complete turn-off of theFETs. Moreover, an FET having a lower threshold voltage Vt can be used,resulting in lower operation voltage.

EXAMPLE 7

FIG. 18 is a diagram of a seventh example of the logic circuit accordingto the present invention. The logic circuit in FIG. 18 receives D1 andD2 as inputs at nodes 1801 and 1802, performs a NAND operation andoutputs the operation results to node 1834. In other words, only whenboth of nodes 1801 and 1802 are at the L level, node 1834 will becomethe H level. FETs 1830 and 1831 are driven by the voltage convertershown in FIG. 5B. FETs 1832 and 1833 are driven by the voltage convertershown in FIG. 3. As a result, a capability for driving the mainswitching devices is enhanced, thereby enabling higher operations at alower voltage.

EXAMPLE 8

FIG. 19 is a diagram of a eighth example of the logic circuit accordingto the present invention. The operation of the logic circuit of theeighth example is described below. When node 1901 is at the H level, anFET 1903 is ON state, an FET 1913 is OFF state. Therefore, an FET 1915is ON state and an FET 1905 is OFF state, whereby node 1931 is at the Llevel. Assuming that a voltage generator 1904 generates a voltage of Vs,a voltage supply 1981 supplies a power supply voltage Vdd, and node 1901is supplied with a voltage of Vdd, a capacitor 1902 is charged by avoltage of (Vdd-Vs). A gate of the FET 1916 is supplied with a summationof a voltage of node 1901 (i.e., H level) and a voltage due to electriccharge in a capacitor 1912 (described later).

When node 1901 is at the L level, an FET 1903 is OFF state, an FET 1913is ON state. Therefore, an FET 1915 is OFF state and an FET 1905 is ONstate, whereby node 1931 is at the H level. Assuming that a voltagegenerator 1914 generates a voltage of Vs, a voltage supply 1981 suppliesa power supply voltage Vdd, and node 1901 is at the ground level, acapacitor 1902 is charged by a voltage of -Vs. A gate of the FET 1905 issupplied with a summation of a voltage of node 1901 (i.e., L level) andvoltage due to electric charge in a capacitor 1902.

In both of above cases, a gate of any ON-state FET of the FETs 1905 and1915 is supplied with a higher voltage than Vdd, and a gate of anyOFF-state FET of the FETs 1905 and 1915 is supplied with a lower voltagethan the ground level. As a result, the operation speed at a lowoperation voltage can be improved. The eighth example utilizes a simplecircuit configuration, resulting in reduction of a chip area in anintegrated circuit.

In the first to the eighth examples of the logic circuit according tothe present invention, an SOI FET is preferably used as an FET in thevoltage converter for the reasons discussed below. FIG. 20 is a diagramillustrating a structure of an N-channel MOS FET and its equivalentcircuit. FIG. 21 is a diagram illustrating a structure of a P-channelMOS FET and its equivalent circuit. A MOS FET, as shown in FIGS. 20 and21, has a junction capacitor CJ and diodes between the drain and thebackgate, and between the source and the backgate.

FIG. 22A in a diagram illustrating a part of the first example of thelogic circuit, and FIG. 22B is a diagram illustrating a equivalentcircuit shown in FIG. 22A. It is assumed that one terminal of terminalsof a capacitor Cc, which is not dotted, is pulled up to Vdd after thecapacitor Cc is supplied with a voltage of Vdd. A gate voltage Vg willbe represented below using a junction capacitance Cj and a gatecapacitance Cg after electric charge is re-distributed. Electric chargeof Q=VddCc in the initial state is equal to summation of electric charge(Vg-Vdd)Ca charged by the capacitance Cc and electric charge Vg(Cg+2Cj)charged by the capacitance (Cg+2Cj) after the re-distribution, resultingin the following expression:

    VddCc=(Vg-Vdd)Cc+Vg(Cg+2Cj).

Solving for the gate voltage Vg yields:

    Vg=2CcVdd/(Cc+Cg+2Cj).

Therefore, in case of Cc>>Cg+2Cj, the advantage of the present inventioncan be maximized. On the contrary, In case of Cc<<Cg+2Cj, the advantageof the present invention is not so remarkable. Accordingly, settingcircuit constants such that Cc>>Cg+2Cj is satisfied is preferable. Thesmaller the junction capacitance Cj, the more remarkable the advantageof the present invention will become.

In case of Vg≧Vdd+Vf, where Vf represents a forward direction voltage, acurrent flows through the parasitic diodes, whereby the voltage Vg isclamped at Vg=Vdd+Vf. As a result, the gate voltage Vg cannot be pulledup higher than this clamped voltage.

Using a transistor having an SOI structure resolves the above problemdue to the junction capacitance and the parasitic diode, maximizing theadvantages of the present invention. FIG. 23 is a diagram illustrating astructure of an N-channel transistor and a P-channel transistor bothhaving an SOI structure.

Further reasons of high-speed operation enabled in the logic circuitaccording to the present invention are discussed in detail below. Ingeneral, a drain current Id of an FET can be expressed using thefollowing expression:

    Id=β(Vgs-Vt).sup.α,

where α=1.4-2.0, β: a constant, Vgs: gate-source voltage, Vt: thresholdvoltage. A delay time τd of the logic circuit is expressed as follows:

    τd=γCLVdd/Id=γCLVdd/(β(Vgs-Vt).sup.α),

where γ: a constant, CL: a capacitance of a load. Assuming that anoperation speed of a system using the logic circuit is in proportionalwith 1/τd, and that α=2 for the sake of simplicity, the speed f of thesystem is expressed as follows:

    f=1/τd=(β/(γCL))*((Vgs/Vdd).sup.2 -2Vt(Vgs/Vdd)-(Vt/Vdd).sup.2),

where Vgs=ηVdd, η=1+Vov1/Vdd (>1.0). If (Vt/Vdd)² <<1 is assumed, then

    f=(β/(γCL))*(η.sup.2 -2Vtη)=βη(η-2Vt)/(γCL).

Therefore, high operation speed of the system requires a large η and asmall Vt.

On the other hand, a leak current Idleak is expressed as follows;

    Idleak=λw*10.sup.(Vgs-Vt)/s,

where γ: a constant, w: a gate width of a transistor, and S is about 70mV. Substituting Vgs=0 yields

    Idleak=λw*10.sup.(-Vt/s).

As will be appreciated from the above expression, a small leak currentIdleak requires a large threshold voltage Vt.

According to the present invention, the voltage converter increases avoltage supplied to the gate, whereby η will become large, and theoperation speed will be enhanced at an ordinary threshold voltage Vt(i.e., about 0.5 V). Moreover, when the main switching device is OFFstate, the gate is at the ground level, thereby suppressing the leakcurrent.

Furthermore, in the third example, when the main switching device is OFFstate, the gate voltage is -Vov2. In such case, the leak current Idleakis equal to λw*10.sup.(-Vov2-Vt/s). As a result, -Vov2 is about -0.3 V,while Vt is equal to or less than 0.35 V, thereby sufficientlysuppressing the leak current Idleak. This means that a transistor havinga lower Vt can be used as the main switching device. Therefore,increased drive current due to the small Vt overdrives the device,whereby a remarkable advantage of the high operating speed according tothe present invention.

An advantage of the present invention when the logic circuit of theinvention is applied to a transfer gate type switching device will bediscussed below. FIG. 24A is a diagram illustrating a transfer gate typeswitch, and FIG. 24B is a graph showing a conductance Gon between adrain and a source of the transfer gate as a function of a sourcevoltage Vs. In FIG. 24B, Vtn and Vtp are threshold voltages of anN-channel FET and a P-channel FET, respectively. As shown in FIG. 24A,in the transfer gate type switch, voltage potentials of the source andthe drain are not fixed. Therefore the source voltage could be equal tothe power supply voltage Vdd. In such case, in order to turn-on the FET,a voltage higher then (Vdd+Vt) will be necessary. This means that thetransfer gate needs a higher voltage than an ordinary logic gate. FIG.25 is a graph showing a conductance Gon between a drain and a source ofthe transfer gate as a function of a source voltage Vs when the sourcevoltage is low. When the source voltage is about 1.0 V, both of the twocomplementary FETs do not turn-on in a certain region of the sourcevoltage (see hatched region in FIG. 25). The voltage converter of thelogic circuit according to the present invention outputs a voltagehigher than the power supply voltage Vdd to the gate of the mainswitching device, and provides deep backward bias to Vgs when thethreshold voltage Vt of the main switching device is low. As a result,high speed operation at a low operation voltage is possible according tothe present invention.

In the first to the eighth example, a capacitor using a material havinga high dielectric constant can be used as the voltage generator and thecapacitor in the voltage converter. FIG. 26 is a graph showing apolarization of the capacitor of which insulating material is highdielectric constant material as a function of a supplied voltage. Thecapacitor using the high dielectric constant material shows polarizationcharacteristics of FIG. 26. When a voltage supplied to the capacitorusing the high dielectric constant material exceeds a coercive voltageVc, spontaneous polarization will occur. As a result, the capacitorusing the high dielectric constant material retains a certain voltagewithout an external voltage supply. Also, since its dielectric constantis extremely large, a small area provides a large capacitance.Therefore, a voltage drop due to the charge re-distribution is small.Applying this capacitor using the high dielectric constant material tothe logic circuit according to the present invention realizes efficientoperation: a high speed operation at a low voltage.

A comparison of the logic circuit according to the present inventionshown in FIG. 14 to the logic circuit of the prior art will bediscussed. FIG. 27 is a circuit diagram of a logic circuit of the priorart to be compared. FIGS. 28A and 28B are graphs showing voltages Va andVc of the logic circuit of the prior art, and voltages Va and Vc of thelogic circuit of the present invention, respectively at the rising edgeof the output signal. FIGS. 28C and 28D are graphs showing voltages Vband Vc of the logic circuit of the prior art, and voltages Vb and Vc ofthe logic circuit of the present invention, respectively at the fallingedge of the output signal. In FIGS. 28A-28D, the power supply voltageVdd is 1.0 V. Referring to FIG. 28A, according to the prior art, whenthe main switching device is OFF state, the voltage Va is lower than thepower supply voltage Vdd by 0.1 V. As a result, when the main switchingdevice is OFF state, a leak current flows. When the main switchingdevice is ON state, Va is equal to -0.05 V. This results in insufficientdrive capability. In FIG. 28A, the gate delay time is 1.2 ns.

On the other hand, referring to FIG. 28B, when the main switching devicein the present invention is OFF state, Va is equal to Vdd. Therefore,the leak current does not exist. Moreover, when the main switchingdevice is ON state, Vs is equal to -0.3 V. This means the enhanced drivecapability of the present invention. Referring to FIG. 28B, the gatedelay time is 0.9 ns.

Referring to FIG. 28C, in the prior art, Vb is equal to 0.1 V when themain switching device is OFF state, generating the leak current. Vb ishigher than Vdd by only 0.1 V when the main switching device is ONstate. Therefore, the drive power is not sufficient. The gate delay timeis 1.0 ns.

On the other hand, referring to FIG. 28D, Vb is equal to 0 V when themain switching device is OFF state. Therefore, the leak current does notflow. Vb is higher than Vdd by 0.3 V, resulting in high drive power. Thegate delay time is equal to 0.75 ns.

A comparison of the logic circuit according to the present inventionshown in FIG. 29 to the logic circuit of the prior art will bediscussed. FIG. 29 is a circuit diagram of the logic circuit of thepresent invention. As a logic circuit of the prior art, an ordinarycomplementary MOS inverter is used to be compared. A voltage of 0.5 V asthe power supply voltage Vdd is used for the logic circuit of thepresent invention and the prior art. FIG. 30A is a graph showingvoltages Va and Vb of the logic circuit of the present invention at therising edge of the output signal. FIG. 30B is a graph showing a gatevoltage Vg of the main switching device and an output voltage Vout ofthe logic circuit of the prior art at the rising edge of the outputsignal. FIG. 30C is a graph showing voltages Va and Vb of the logiccircuit of the present invention at the falling edge of the outputsignal. FIG. 30D is a graph showing a gate voltage Vg of the mainswitching device and an output voltage Vout of the logic circuit of theprior art at the falling edge of the output signal.

Referring to FIG. 30A, Va is higher than Vdd by 0.5 V when the mainswitching device is OFF state, whereby the main switching device isdeeply cut off. Va is lower than the ground level by 0.3 V, whereby thedrive capability is enhanced. The gate delay time is 1.63 ns. On theother hand, as shown in FIG. 30B, the gate delay time of the prior artis 5.6 ns.

Referring to FIGS. 30C and 30D, also at the falling edge, the gate delaytime of the prevent invention is smaller than that of the prior art.Specifically, the gate delay time of the present invention is only 1.88ns, while the gate delay time of the prior art is 6.4 ns.

As will be apparent from the above described simulation result, thelogic circuit according to the present invention enables enhancing theoperation speed especially in the low operation voltage region.

In the first to the eighth example, a terminal which is not connected tothe node receiving the input signal can be set to a voltage other thanthe power supply voltage of Vdd and the ground level. For example, inFIG. 13, the voltage generators 1313 and 1323 are provided in additionto the power supply 1380. A voltage which is generated by the voltagegenerators 1313 and 1323 is not limited to Vdd. Alternatively, the powersupply 1380 can be used in place of the voltage generators 1313 and1323. Specifically, the contact, which is connected to the negativeelectrode of the voltage generator 1313, can be connected to the ground,and the contact, which is connected to the positive electrode of thevoltage generator 1323, can be connected to the positive electrode ofthe power supply 1380. As will be appreciated, this modification of theabove voltage potential is within the scope of the present invention.

In the logic circuit according to the present invention, a terminalother than the control terminal (i.e., is drain or a source in case ofan FET) can be pulled up when necessary. For example, although in FIG.2A, node 222 of the main switching device 220 is in a floating state,node 222 can be pulled up to the power supply voltage Vdd in an actualcircuit. Alternatively, without connecting node 223 to the ground, nodes222 and 223 can be used as output terminals.

In the logic circuit according to the present invention, a drain voltageand a source voltage of an FET included in the voltage converter areforward direction voltages with respect to a substrate of thetransistor. When the power supply voltage is equal to or lower than 1.0V, however, the drain voltage and the source voltage do not cause a deepforward bias. Moreover, this forward bias can be avoided if thesubstrate of each transistor is controlled such that, for example, asubstrate of a P-channel MOS FET (i.e., an N-well) is at a highervoltage potential then the power supply voltage, and a substrate of aN-channel MOS FET (i.e., a P-well) is at a lower voltage potential thanthe ground. Also, floating the substrate using the SOI technology willavoid the forward bias. In general, as the voltage of the drain or thesource will be at a higher voltage with respect to the transistorsubstrate, a voltage is supplied between the drain and the source whenthe transistor constituting the switching circuit is OFF state, therebycausing an off-leak current between the drain and the source. In thelogic circuit according to the present invention, transistors forswitching are used, and a gate width thereof is small. Therefore, thepresent invention does not suffer from the problem due to the off-leakcurrent. Alternatively, the off-leak current can be avoided by settingthe threshold voltage to a high value.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

What is claimed is:
 1. A logic circuit comprising:a main switching meansfor changing conduction state between at least two terminals inaccordance with a voltage supplied to a control terminal; and a voltageconverting means for converting a voltage at an input terminal andoutputting the converted voltage to the control terminal, the voltageconverting means including a voltage generating means and a subswitching means, wherein the state of the logic circuit is either afirst state in which the main switching means is non-conductive or asecond state in which the main switching means is conductive, andwherein, in the first state, the voltage converting means converts thevoltage at the input terminal into a converted voltage at which the mainswitching means is cut off sufficiently, and particularly the voltageconverting means converts the voltage at the input terminal into theconverted voltage which is outputted to the control terminal, theconverted voltage being operative to drive the main switching meansfurther into a non-conductive state as compared to if the voltage at theinput terminal itself was outputted to the control terminal, the voltagegenerating means includes a capacitor of which a first terminal isconnected to the input terminal; and the sub switching means:connects asecond terminal of the capacitor to the control terminal of the mainswitching means in the first state; and connects the first terminal ofthe capacitor to the control terminal of the main switching means, andconnects the second terminal of the capacitor to a around in a stateother than the first state.
 2. A logic circuit comprising:a mainswitching means for changing conduction state between at least twoterminals in accordance with a voltage supplied to a control terminal;and a voltage converting means for converting a voltage at an inputterminal and outputting the converted voltage to the control terminal,the voltage converting means including a voltage generating means and asub switching means, wherein the state of the logic circuit is either afirst state in which the main switching means is non-conductive or asecond state in which the main switching means is conductive, andwherein, in the first state, the voltage converting means converts thevoltage at the input terminal into a converted voltage at which the mainswitching means is cut off sufficiently, the voltage generating meansincludes a capacitor of which a first terminal is connected to the inputterminal; and the sub switching means:connects a second terminal of thecapacitor to the control terminal of the main switching means in thefirst state; and connects the first terminal of the capacitor to thecontrol terminal of the main switching means, and connects the secondterminal of the capacitor to a ground in a state other than the firststate.
 3. A logic circuit according to claim 2, wherein the mainswitching means includes an N-channel MOS FET.
 4. A logic circuitaccording to claim 2, wherein the main switching means includes aP-channel MOS FET.